Colonization of a Water System by Legionella Organisms and Nosocomial Legionellosis: A 5-Year Report From a Large Italian Hospital

we look back again at the cleanroom analogy, the international standard for cleanrooms details test methods and equipment for airflow volumes, installed filter leakage, air exchange rate (ie, recovery time) and containment, in addition to pressure differentials and airflow visualization. The recent design manual for hospitals published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers points out that "maintaining a negative air pressure between the All (airborne infectious isolation room) and the corridor may not be enough to provide isolation" and "the truly significant factor in determining the amount of air volume migration from the room to the corridor is the airflow volume differential" and that it is necessary to "maintain a specific differential airflow rate" in an isolation room. How can one be certain of maintaining a specific differential airflow rate if it is not periodically measured? Airflow volume differential is dependent on envelope tightness and pressure differential. Without a sufficiently tight envelope for the isolation room, pressure differentials cannot be maintained and airflow direction cannot be controlled. The isolation room guideline from Norway recognizes this and explicitly calls for envelope tightness testing as part of isolation room commissioning. Unfortunately, the guideline doesn't say how to do the test nor does it give any indication as to what value is acceptable for an envelope tightness test result for an isolation room.